The present invention relates to a semiconductor laser having an improved resonator reflecting plane.
A conventional semiconductor laser having a double heterojunction structure is known to have a low operation current density, long life, and good emission characteristics. Therefore, such a semiconductor laser is used as a key device in optoelectronic industries such as communications and computers. In order to widen the application scope of the semiconductor laser, problems such as an improvement in a laser output, lower cost, or the like must still be resolved.
A conventional double heterostructure semiconductor laser comprises a three-layered structure in which an active layer formed of III-V compound semiconductor such as gallium arsenide (GaAs) is sandwiched between two GaAlAs layers. This three-layered structure is further sandwiched between positive and negative electrodes. In a resonator of a semiconductor laser having such a structure, two reflecting planes or mirrors are obtained by cleaving a double heterostructure wafer. They are perpendicular to a junction plane. Since these cleaved surfaces are significantly smooth, they can be used as the reflecting planes of the resonator.
In such a structure, when a forward-bias DC voltage is applied between the positive and negative electrodes, minority carriers are injected into a GaAs active layer where light emission takes place. Since a refractive index of the GaAs active layer is higher than that of the GaAlAs layers formed on the upper and lower surfaces thereof, emitted light is confined to the active layer and is repeatedly reflected between two opposite reflecting planes, thereby causing laser emission. Part of the laser light is externally emitted through the reflecting planes.
In a conventional semiconductor laser, the cleavage process is responsible for low-reproducibility and poor yield because of its difficulty to obtain good reflecting planes and a pre-designed laser cavity. Furthermore, the cleaved surface has a low reflectance of about 30%, thereby preventing an improvement in the output of a semiconductor laser. This is because low reflectance decreases optical feedback gain.
Meanwhile, in the manufacturing of a surface emitting semiconductor laser, a method of forming reflecting planes utilizing a combination of epitaxial growth and chemical etching in place of cleaving has been proposed. In such a laser the reflectance of the obtained reflecting plane is as low as 15%, too low for practical use. Furthermore, in the surface emitting semiconductor laser, the cavity length of the resonator is very short, e.g., about 10 .mu.m. Therefore, if the positions of the reflecting planes cannot be controlled with high precision, on the order of several tens of angstroms, mass production of the surface emitting semiconductor lasers cannot be performed. No available method which satisfies these conditions has been developed.